is it possible to put d.e. in a sand system?we are having a problem with water down here in mexico turning the pools green.not algae which we have blooms later in the year but from the water comin from our city source or from wells.i have heard it is phosphates or other metals,that are to small to get flitered out buy the sand.someone suggested using d.e. to coat the sand.will ir harm the system?thanks for any help

The green is most likely from metals in the water and you could try using a metal sequestrant (HEDP-based is preferable). The fact that you get algae later in the season is most likely due to your continued use of Trichlor pucks/tabs which build up Cyanuric Acid (CYA) in the water. The following are chemical facts independent of concentration of product or of pool size:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm.

So at 2 ppm FC daily usage, the CYA level would rise by over 35 ppm per month making the chlorine less effective unless you were to proportionately raise the FC level to compensate. I suggest you read the Pool School to learn how to properly maintain the pool. You will notice that there is an article on how to Add DE to a Sand Filter, but I do not believe that will help your particular situation unless the green color is from fine particulate matter.